In many application domains, measurement of the pH of a solution or substance is important. In the medical field, the measurement of the pH of a patient's blood can give valuable insights into the condition of the patient because pH values that deviate from expected values can indicate that the patient is suffering from some medical condition. pH sensors used in the medical field are typically costly because they must comply with health and safety regulations when blood samples are returned to the patient, and must be highly accurate.
In the food industry, the measurement of the pH of a food product, e.g. dairy products and meat, is often used because the pH is indicative of the condition of the food product. In such application domains, a high volume of measurements may have to be taken, either because of the large volumes of food or because the duration of the time period over which the food is stored. In such application domains, large numbers of (disposable) sensors may be required, which therefore requires the sensors to be cheap.
As is known, one of the main problems with the stability of pH sensors is that the reference electrode potential may not be stable over time, for instance because ion concentrations in a buffer solution to maintain the reference electrode potential leak away. Such buffer solutions may for instance be employed to protect the reference electrode from variations in the same ion concentration in the medium to be analyzed.
For instance, a commonly used reference electrode is a silver chloride electrode in which the reference potential E is defined by the Nernst equation:
  E  =            E      0        -                  RT        F            ⁢      ln      ⁢                          ⁢              a                  Cl          -                    
In this equation, E0 is the standard electrode potential, R is the ideal gas constant, T is the temperature, F is Faraday's constant and aCl− is the activity or effective concentration of the chloride ions. It will be immediately recognized that a change in the effective chloride concentration has a direct impact on the reference electrode potential E. Many silver chloride reference electrodes include a solution containing Cl− ions that needs periodic replenishing in order to maintain a stable reference potential.
This approach may not be suitable in application domains where the sensors are operated by staff that are not trained to replenish the reference electrode or perform recalibrations. Moreover, recalibration and refilling may be too expensive or cannot be performed at all for specific process applications, e.g. monitoring of food quality during transport.
Miniaturized pH sensors including reference electrodes have been made in which a plurality of sensors including a pH sensor and a reference sensor have been integrated on a single chip. However, the manufacturing processes of such ICs is rather complex, such that the cost of such devices is prohibitive for low-end application domains such as the food industry and water and soil quality probes.